1. The Field of the Invention
The present invention relates to a method and an apparatus for producing glass, in which the reduction of reduction-sensitive components of the glass is decreased and preferably is avoided during the melting and fining process. The glass is preferably glass with a high refractive index. According to the invention the term “reduction-sensitive” means sensitivity to both reduction and oxidation reactions, i.e. “reduction-sensitive” also means “redox-sensitive”.
2. The Description of the Related Art
Many high-index materials and glass, especially those used for optical, fiber-optic and display-applications as well as for applications for protection and passivation of electronic parts and components, are composed so that the melt technical production leads to a considerable loss of performance in conventional systems and facilities concerning very important properties for the particular application such as transmission, refractive index position, uniformity, electrical resistance and compressibility, thus rendering the production uneconomical.
Such materials and glass compositions comprise components that are reduction sensitive and/or corrosive in the molten state. Reduction sensitive, so-called polyvalent, components can have different redox states (oxidation states) in the melt. The equation for the redox equilibrium of such a component is:
wherein M(x+n)+ is the oxidized form and Mx+ the reduced form of the species M. The redox partners are usually oxygen ions (O2−) present in the melt and oxygen dissolved in the melt (O2).
For this redox equilibrium and under the precondition that the oxygen anion concentration is constant (O2−=const.) the equilibrium constant K is given by equation (1):K=([Mx+]·[O2]n/4]/([M(x+n)+])  (1).
From equation (1) and equation (2), ΔH−T*ΔS=−RT*ln K, the following dependency of the redox equilibrium concentration ratio [M(x+n)+]/[Mx+], [Ox]/[Red], respectively, on the temperature T and on the oxygen concentration [O2] results:ln([M(x+n)+]/[Mx+])=ΔH/(R·T)−ΔS/R+(n/4)·ln [O2]  (3)wherein ΔH=enthalpy of the reaction, ΔS=entropy of the reaction, R=specific gas constant.
The outcome of this is that the redox equilibrium is shifted towards the reduced species Mx+, if the temperature T rises and/or the oxygen concentration [O2] decreases.
With decreasing temperature T and/or rising oxygen concentration [O2] the redox equilibrium is shifted towards the oxidized species M(x+n)+.
The redox relationship of the oxidized form and reduced form of a component at a distinct temperature and a distinct oxygen concentration is finally determined by the composition of the melt, the substance and matrix specific thermodynamic variables (ΔH and ΔS) and the possible redox reactions with other polyvalent components. For example in a melt of the composition (% by weight): 8.8% Na2O; 29.6% SrO; 61.1% P2O5 and 0.5% SnO2 at 1200° C. and with an oxygen partial pressure of 0.21 bar (this is the partial pressure in the atmosphere) about 94% of the tin are present in the form of Sn4+ (oxidized form), whereas only 6% are present in the form of Sn2+ (reduced form). If the temperature is increased to 1500° C. (at unchanged oxygen concentration i.e. unchanged partial pressure), the redox relationship is changed. In that case the thermodynamic equilibrium shifts so that 47% of the tin are present as Sn4+ (oxidized form), 50% are present as Sn2+ (reduced form) and already 3% are present as elemental metallic tin. If the oxygen concentration is elevated i.e. the partial pressure is increased to 1 bar at 1500° C., 57.5% of the tin are present as Sn+ (oxidized form), 41% are present as Sn2+ (reduced form) and only 1.5% are present as elemental metallic tin. The phosphate ions in this melt underlie the thermodynamic redox equilibrium, too. At 1200° C. and an oxygen partial pressure of 0.21 bar (this is the partial pressure in the atmosphere) about 99.9% of the phosphorus are present as P5+ (oxidized form) and only 0.1% as P3+ (reduced form). At a temperature of 1500° C. and reducing conditions, for example at an oxygen partial pressure of 10−5 bar, about 89% of the phosphorous are present in the form of P5+ (oxidized form), but already 11% are present as P3+ (reduced form) and 0.1% are even present as elemental phosphorous (source: “Das Redoxverhalten polyvalenter Elemente in Phosphatschmelzen und Phosphatgläsern”, Dissertation Annegret Matthai, Jena 1999).
Critical for the product properties of materials to be produced are, in connection with the reduction of the components in the materials, a direct effect (decrease) on the optical transmission values due to the reduced species themselves on the one hand and an indirect effect (decrease) on the optical transmission values due to reaction of reduced species with container materials. Furthermore important properties of the materials, such as electrical resistance and the dielectric strength, are influenced negatively, but the reduced species or their corrosion products also influence the crystallization and molding properties.
The reducible species will directly influence the transmission properties if these materials are not present in their highest possible oxidation state. High oxidation states normally have electron configurations that forbid electron transitions due to absorption of light in the visible spectral region, which influences the optical transmission of the material. But in case these components are present in lower oxidation states, electron configurations may occur that allow electron transitions. These lead to absorption of light in the visible spectral region and, thus, to discoloration. Such so-called polyvalent components are, for example: niobium, phosphorous, vanadium, titanium, tin, molybdenum, tungsten, lead and bismuth.
If these components are further reduced thermally or chemically, they can have an oxidation state of 0 and, hence, be present in elemental form. Precipitation of particles and/or crystals in the nanometer range occurs. This leads under the influence of light to diffraction and scattering effects in the material that influence the transmission in the visible spectral range, too. But other properties like the electrical resistance, the dielectric strength and the crystallization properties can also be influenced.
If the precipitated particles or crystals grow, tension and defects occur in the material that can during irradiation with high energy-densities (for example: lasers) lead to destruction of the glass. As described in DE 101 38 109 A1 such particles must be oxidized again through elaborate processes, for example using highly toxic gaseous chlorine, in order to ameliorate the optical properties of the glass after the melting process. The addition of nitrates in the glass batch that provides for strongly oxidizing conditions in the melt by liberation of NO2 and other nitrous gases has to be rejected based on environmental and working security grounds. The described process is also highly dangerous in connection with free phosphate (P2O5), because it can lead to explosive reactions.
Components that can thermally and/or chemically be reduced to the elemental state in the melt are for example phosphorous, tin, germanium, lead, arsenic, antimony, molybdenum, bismuth, silver, copper, platinum metals and gold.
If there is an affinity or a tendency towards alloy formation between the components reduced in the melt and the container material, the reduced components alloy with the container material and are thus continuously extracted from the melt by chemical equilibrium, the formation of which would lead to an abating of the reaction. So a cycle is set up that in the end leads to a destruction of the crucible, because of the alloy formation the resistance and the melting point of the crucible materials is strongly decreased. This is especially critical in case of crucibles of the platinum group. For example, the alloying of 5% phosphorous with platinum leads to a decrease in melting point from 1770° C. to 588° C. with the resulting effects on the durability of the crucible.
In a less dramatic case the in situ formed alloy is at once dissolved in the melt and a large amount of crucible material is introduced into the melt occurs. In the case of platinum elements this is connected with a discoloration and a worsening of the transmission properties.
It is especially critical in order to achieve high refractive indexes of nd>1.7, preferably nd>1.7 5, and/or minimum possible softening temperatures that are of great importance for precision and precise pressing, that high amounts of reducible compounds are introduced into the materials and glasses.
The use of so-called flameproof materials having an oxidic or oxidic-ceramic basis, for example, zirconium, silicate, or aluminium oxide material, only solves a part of the above-described problems and is additionally not an economically reasonable solution, either. These materials are indeed not reducing, do not show any alloy forming tendencies towards elemental precipitations and are relatively stable toward many melt compositions as far as corrosion and dwell time are concerned. But when they are attacked by the melt they dissolve in part and are “bad-natured”, that is these flameproof materials can lead to faults in the glass.
Especially aggressive attack of high-index melt compositions that should additionally be workable by precise pressing is not acceptable, because the dissolution of the crucible and the entry of the material into the melt lead to unwanted changes in properties of the materials and glasses, especially an increase in the transformation temperature, changes in viscosity properties, changes in refractive index and Abbe number as well as changes in transmission. Furthermore areas are formed that are enriched with the flameproof material, which become visible because of striae and refractive index changes in the material.
As a further effect of the strong aggressive attack on flameproof materials, apart from the considerable worsening of properties and uniformity of the glass, an in part extreme shortening of the dwell time of the melt equipment arises causing extensive costs. On the one hand costs arise because of the need to renew the melt unit and on the other hand because of repeated downtime costs.
The continuous melting and fining of corrosive materials and glass in systems with cooled walls on which the material freezes and forms a contact area of specific material is well known for many technical and optical glasses and patented, too (DE 102 44 807 A1, DE 199 39 779 A1, DE 101 33 469 A1). The container, in which the melt is heated and in which the fining process takes place, usually comprises meander-shaped cooling circuits and usually high-frequency radiation is used for the heating of the melt. The forming boundary layer of specific material to a large extent prevents the attack of the melt on the wall material. Hence no contamination of the melt from wall material takes place. All these inventions claim, inter alia, the melting and in part the fining of corrosive, optical glasses of high purity in these so-called skull-devices. Because of the high reduction potential and the comparatively high temperatures that are needed for injection and melting of high-index melts, especially in the niobium oxide/phosphorous oxide system, none of the cited documents offers the potential to produce the claimed glasses in the necessary quality and with the necessary properties.